This invention pertains generally to sputter coating and, more particularly, to an apparatus and a method for depositing a thin film coating on a workpiece by collimated sputtering.
Sputter coating is commonly employed in the formation of films on substrates in the manufacture of semiconductor devices. Planar magnetrons have long been used as sputtering devices to coat silicon wafers with various materials during the manufacture of integrated circuits. While aluminum has traditionally been the most common metallization material used in integrated circuits, other sputtered materials, such as titanium, titanium nitride, and titanium/tungsten alloy are now also commonly being used. Sputtering is also used during semiconductor manufacturing to deposit various non-metallic compounds either by direct sputtering or by reactive sputtering. Some of these non-metallic compounds, such as titanium nitride, are sufficiently conductive to be used in interconnect structures.
As integrated circuit densities have increased and device geometries have correspondingly shrunk, it has become more difficult to use sputter coating to form a uniform thin film, or step coating, which conforms to the shape of the surface of the workpiece or substrate where a step occurs, particularly at the upper or lower corner of an opening such as a hole or a via in the surface of the workpiece. It is also difficult to fill small openings (e.g., now commonly one micron, or less, in diameter or width) and to provide controlled film growth on the side and bottom walls of such openings. These difficulties arise because the sputtered atoms tend to leave the source in all directions, then collide with each other and scatter, arriving at the workpiece from a variety of angles. Particles (atoms) which arrive at angles much different than normal to the substrate surface tend to produce lateral growth on the surface. This lateral growth can result in overgrowth at the tops of the openings which can eventually close off via openings before they can be properly coated, resulting, for example, in an inadequate electrical connection between integrated circuit layers.
One approach to overcoming the foregoing problem is to use a collimation filter to limit the angles with which sputtered atoms strike the workpiece. The theory of collimation is that if, on average, the angles of incidence of sputtered particles are close to normal, then more of the atoms will penetrate to the bottoms of via holes, so that a conformal film will be formed in the via and there will be little or no overgrowth at the top of the via. The collimating filter typically comprises a plurality of cells in an array which is interposed between a sputter source and a workpiece. To reach the substrate, atoms emitted from the sputter source must pass through the cells without striking the cell walls, i.e., the cell walls intercept sputtered particles travelling at angles which are not close to being normal to the substrate. This causes a substantial amount of sputtered material to deposit on the filter itself.
However, there have been several problems associated with prior art sputtering systems employing collimation filters. First, unless the sputter source is very highly uniform, there will be an uneven film distribution on the surface of the wafer. Most sputter sources, particularly those which are used with the large semiconductor wafers common in the industry today (e.g., wafers having an eight-inch diameter), do not have a sufficiently uniform emission distribution to work with a collimation filter. These sputter sources rely on the scattering and angularity of sputtered particles to compensate for the fact that atoms are not emitted at a uniform rate from all points on the sputter target. For example, most prior art sputtering sources do not provide uniform erosion at the center of the target. However, a collimating filter negates the compensating effects of angularity and scattering. Thus, if a sputter source does not emit particles from its center, then an adequate layer of film will not be deposited at the center of the wafer.
Second, most sputter sources operate at pressures at which particle scattering is a significant effect over short distances. At the pressures commonly employed, the mean-free-path of sputtered atoms is relatively short. Accordingly, any directionality imparted to the particle flux by the collimation filter will be lost over a relatively short distance due to scattering. One attempted solution to this problem has been to place the collimation filter very close to the surface of the substrate to minimize scattering effects. However, this xe2x80x9csolutionxe2x80x9d causes a shadowing effect such that the pattern of collimation filter cell walls is noticeably present on the surface of the wafer. Some prior art discloses means for moving either the filter or the wafer to avoid the shadowing effect. Movement of the wafer over a large area can also help overcome the aforementioned problem of non-uniformity. However, wafer movement is undesirable due to the added complexity it creates and the increased likelihood of contamination associated with moving parts within the sputter chamber.
Another problem with prior art sputtering sources using collimation to fill small diameter, high aspect ratio vias is that there has been no recognition of the need to optimize the aspect ratio of the collimation filter so that there is a desired balance between the deposition on the sidewall of the via and the bottom of the via. If the aspect ratio of the collimator cells is too high, there will be too little sidewall deposition. on the other hand, if the aspect ratio of the collimator cells is too low the sputtered atoms will behave much as if there is no filter with the same result, e.g., undesired film build-up and overhang at the corners of the via which blocks film from reaching the via bottom.
Yet another problem with prior art sputtering systems using collimation is that much of the sputtered material is deposited on the walls of the collimation filter rather than on the substrate. Not only does this reduce the deposition rate, thereby impairing wafer xe2x80x9cthrough-putxe2x80x9d, but also has a tendency to increase the amount of particulate material in the sputter chamber. As a film of sputtered material is deposited on the cell walls the material tends to xe2x80x9cflakexe2x80x9d off. Some of the large particles thus formed end up on the surface of the wafer, where they can destroy the integrated circuit chips being fabricated. Semiconductor manufacturers go to great lengths to minimize the presence of extraneous particles in the wafer processing environment. It should also be noted that moving the wafers or collimation filter, as described above, can exacerbate the problem of extraneous particle creation.
The inventors of this invention have determined that the problem of extraneous particle creation is, in part, related to the mismatch in the thermal coefficients of expansion (TCE""s) between the material used to construct the collimation filter and the material being sputtered. The filter, and the film deposited on it, go through substantial thermal cycling as the sputtering equipment is used. For example, a collimating filter may initially be at room temperature when the system is started, and then be heated to several hundred degrees centigrade as the system is operated.
The inventors have also recognized that the degree to which thermal mismatch causes problems is dependent on the properties of the material being deposited and the material used to construct the filter. The problem is particularly acute when depositing non-metallic materials which tend to have low ductility and are more prone to flaking. Most metallic films have sufficiently high ductility and adhesion so that they can tolerate appreciable mismatch in TCE between the film and the collimator. On the other hand, many non-metallic materials that are commonly deposited by sputtering, such as titanium nitride, are brittle and prone to flaking.
Most known prior art collimating filters have been constructed out of stainless steel. When sputtering aluminum, which has heretofore been the most common metallization layer used in semiconductor manufacture, extraneous particle creation has not been unacceptably poor. However, when sputtering films of newer materials, such as titanium, titanium nitride, or titanium/tungsten alloy, particle creation becomes unacceptable when using a collimation filter made of stainless steel. It is noted that these newer materials are finding increasing use as device geometries shrink and, hence, the need for the benefits of collimation increases.
It is in general an object of the present invention to provide a new and improved collimated sputtering apparatus for coating a workpiece such as a semiconductor wafer.
Another object of the invention is to provide apparatus of the above character which overcomes the limitations and disadvantages of sputtering systems heretofore provided.
Another object of the invention is to provide apparatus of the above character which is particularly suitable for use in thin films of sputtered titanium, titanium nitride and titanium/tungsten alloy.
It is yet another object of the present invention to overcome the problems which have, to date, been associated with the use of collimation filters in sputtering systems.
These and other objects are achieved in accordance with the invention by supporting a workpiece in a chamber, emitting particles from a sputter source substantially uniformly throughout an area of greater lateral extent than the workpiece, passing the particles through a particle collimating filter having a plurality of transmissive cells with an optimized length to diameter ratio positioned between the source and the workpiece to limit the angles at which the particles can impinge upon the workpiece, and maintaining the pressure within the chamber at a level which is sufficiently low to prevent substantial scattering of the atoms between the source and the workpiece. In the preferred embodiment of the present invention, the collimation filter is made of titanium or other material which is compatible with the material being deposited by sputtering.